LLVM 19.0.0git
DemoteRegToStack.cpp
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1//===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "llvm/ADT/DenseMap.h"
10#include "llvm/Analysis/CFG.h"
11#include "llvm/IR/Function.h"
15using namespace llvm;
16
17/// DemoteRegToStack - This function takes a virtual register computed by an
18/// Instruction and replaces it with a slot in the stack frame, allocated via
19/// alloca. This allows the CFG to be changed around without fear of
20/// invalidating the SSA information for the value. It returns the pointer to
21/// the alloca inserted to create a stack slot for I.
23 std::optional<BasicBlock::iterator> AllocaPoint) {
24 if (I.use_empty()) {
25 I.eraseFromParent();
26 return nullptr;
27 }
28
29 Function *F = I.getParent()->getParent();
30 const DataLayout &DL = F->getParent()->getDataLayout();
31
32 // Create a stack slot to hold the value.
33 AllocaInst *Slot;
34 if (AllocaPoint) {
35 Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
36 I.getName()+".reg2mem", *AllocaPoint);
37 } else {
38 Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
39 I.getName() + ".reg2mem", F->getEntryBlock().begin());
40 }
41
42 // We cannot demote invoke instructions to the stack if their normal edge
43 // is critical. Therefore, split the critical edge and create a basic block
44 // into which the store can be inserted.
45 if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
46 if (!II->getNormalDest()->getSinglePredecessor()) {
47 unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
48 assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
49 BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
50 assert(BB && "Unable to split critical edge.");
51 (void)BB;
52 }
53 } else if (CallBrInst *CBI = dyn_cast<CallBrInst>(&I)) {
54 for (unsigned i = 0; i < CBI->getNumSuccessors(); i++) {
55 auto *Succ = CBI->getSuccessor(i);
56 if (!Succ->getSinglePredecessor()) {
57 assert(isCriticalEdge(II, i) && "Expected a critical edge!");
58 [[maybe_unused]] BasicBlock *BB = SplitCriticalEdge(II, i);
59 assert(BB && "Unable to split critical edge.");
60 }
61 }
62 }
63
64 // Change all of the users of the instruction to read from the stack slot.
65 while (!I.use_empty()) {
66 Instruction *U = cast<Instruction>(I.user_back());
67 if (PHINode *PN = dyn_cast<PHINode>(U)) {
68 // If this is a PHI node, we can't insert a load of the value before the
69 // use. Instead insert the load in the predecessor block corresponding
70 // to the incoming value.
71 //
72 // Note that if there are multiple edges from a basic block to this PHI
73 // node that we cannot have multiple loads. The problem is that the
74 // resulting PHI node will have multiple values (from each load) coming in
75 // from the same block, which is illegal SSA form. For this reason, we
76 // keep track of and reuse loads we insert.
78 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
79 if (PN->getIncomingValue(i) == &I) {
80 Value *&V = Loads[PN->getIncomingBlock(i)];
81 if (!V) {
82 // Insert the load into the predecessor block
83 V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
84 VolatileLoads,
85 PN->getIncomingBlock(i)->getTerminator()->getIterator());
86 Loads[PN->getIncomingBlock(i)] = V;
87 }
88 PN->setIncomingValue(i, V);
89 }
90
91 } else {
92 // If this is a normal instruction, just insert a load.
93 Value *V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
94 VolatileLoads, U->getIterator());
95 U->replaceUsesOfWith(&I, V);
96 }
97 }
98
99 // Insert stores of the computed value into the stack slot. We have to be
100 // careful if I is an invoke instruction, because we can't insert the store
101 // AFTER the terminator instruction.
102 BasicBlock::iterator InsertPt;
103 if (!I.isTerminator()) {
104 InsertPt = ++I.getIterator();
105 // Don't insert before PHI nodes or landingpad instrs.
106 for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
107 if (isa<CatchSwitchInst>(InsertPt))
108 break;
109 if (isa<CatchSwitchInst>(InsertPt)) {
110 for (BasicBlock *Handler : successors(&*InsertPt))
111 new StoreInst(&I, Slot, Handler->getFirstInsertionPt());
112 return Slot;
113 }
114 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
115 InsertPt = II->getNormalDest()->getFirstInsertionPt();
116 } else if (CallBrInst *CBI = dyn_cast<CallBrInst>(&I)) {
117 for (BasicBlock *Succ : successors(CBI))
118 new StoreInst(CBI, Slot, Succ->getFirstInsertionPt());
119 return Slot;
120 } else {
121 llvm_unreachable("Unsupported terminator for Reg2Mem");
122 }
123
124 new StoreInst(&I, Slot, InsertPt);
125 return Slot;
126}
127
128/// DemotePHIToStack - This function takes a virtual register computed by a PHI
129/// node and replaces it with a slot in the stack frame allocated via alloca.
130/// The PHI node is deleted. It returns the pointer to the alloca inserted.
131AllocaInst *llvm::DemotePHIToStack(PHINode *P, std::optional<BasicBlock::iterator> AllocaPoint) {
132 if (P->use_empty()) {
133 P->eraseFromParent();
134 return nullptr;
135 }
136
137 const DataLayout &DL = P->getModule()->getDataLayout();
138
139 // Create a stack slot to hold the value.
140 AllocaInst *Slot;
141 if (AllocaPoint) {
142 Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
143 P->getName()+".reg2mem", *AllocaPoint);
144 } else {
145 Function *F = P->getParent()->getParent();
146 Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
147 P->getName() + ".reg2mem",
148 F->getEntryBlock().begin());
149 }
150
151 // Iterate over each operand inserting a store in each predecessor.
152 for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
153 if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
154 assert(II->getParent() != P->getIncomingBlock(i) &&
155 "Invoke edge not supported yet"); (void)II;
156 }
157 new StoreInst(P->getIncomingValue(i), Slot,
158 P->getIncomingBlock(i)->getTerminator()->getIterator());
159 }
160
161 // Insert a load in place of the PHI and replace all uses.
162 BasicBlock::iterator InsertPt = P->getIterator();
163 // Don't insert before PHI nodes or landingpad instrs.
164 for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
165 if (isa<CatchSwitchInst>(InsertPt))
166 break;
167 if (isa<CatchSwitchInst>(InsertPt)) {
168 // We need a separate load before each actual use of the PHI
170 for (User *U : P->users()) {
171 Instruction *User = cast<Instruction>(U);
172 Users.push_back(User);
173 }
174 for (Instruction *User : Users) {
175 Value *V =
176 new LoadInst(P->getType(), Slot, P->getName() + ".reload", User->getIterator());
178 }
179 } else {
180 Value *V =
181 new LoadInst(P->getType(), Slot, P->getName() + ".reload", InsertPt);
182 P->replaceAllUsesWith(V);
183 }
184 // Delete PHI.
185 P->eraseFromParent();
186 return Slot;
187}
MachineBasicBlock MachineBasicBlock::iterator DebugLoc DL
This file defines the DenseMap class.
iv Induction Variable Users
Definition: IVUsers.cpp:48
#define F(x, y, z)
Definition: MD5.cpp:55
#define I(x, y, z)
Definition: MD5.cpp:58
uint64_t IntrinsicInst * II
#define P(N)
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
an instruction to allocate memory on the stack
Definition: Instructions.h:60
LLVM Basic Block Representation.
Definition: BasicBlock.h:60
InstListType::iterator iterator
Instruction iterators...
Definition: BasicBlock.h:166
CallBr instruction, tracking function calls that may not return control but instead transfer it to a ...
A parsed version of the target data layout string in and methods for querying it.
Definition: DataLayout.h:110
Invoke instruction.
An instruction for reading from memory.
Definition: Instructions.h:173
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1209
An instruction for storing to memory.
Definition: Instructions.h:289
bool replaceUsesOfWith(Value *From, Value *To)
Replace uses of one Value with another.
Definition: User.cpp:21
LLVM Value Representation.
Definition: Value.h:74
#define llvm_unreachable(msg)
Marks that the current location is not supposed to be reachable.
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ)
Search for the specified successor of basic block BB and return its position in the terminator instru...
Definition: CFG.cpp:79
auto successors(const MachineBasicBlock *BB)
AllocaInst * DemoteRegToStack(Instruction &X, bool VolatileLoads=false, std::optional< BasicBlock::iterator > AllocaPoint=std::nullopt)
This function takes a virtual register computed by an Instruction and replaces it with a slot in the ...
AllocaInst * DemotePHIToStack(PHINode *P, std::optional< BasicBlock::iterator > AllocaPoint=std::nullopt)
This function takes a virtual register computed by a phi node and replaces it with a slot in the stac...
BasicBlock * SplitCriticalEdge(Instruction *TI, unsigned SuccNum, const CriticalEdgeSplittingOptions &Options=CriticalEdgeSplittingOptions(), const Twine &BBName="")
If this edge is a critical edge, insert a new node to split the critical edge.
bool isCriticalEdge(const Instruction *TI, unsigned SuccNum, bool AllowIdenticalEdges=false)
Return true if the specified edge is a critical edge.
Definition: CFG.cpp:95